Functional fluids

ABSTRACT

Production of functional fluids for aircraft, industrial and marine use, having good fire resistance, desirable viscosity characteristics, low density and which are relatively inexpensive, especially useful as aircraft hydraulic fluids, comprising a mixture of a phosphorus ester, such as a phosphate, e.g., tributyl phosphate, and a diester of adipic acid or sebacic acid, such as diisodecyl adipate.

United States Patent [1 1 Sheratte Nov. 19, 1974 [73] Assignee: Douglas McDonnell Corporation,

Santa Monica, Calif.

[22] Filed: Mar. 29, 1971 [21] Appl. No.: 129,269

[52] US. Cl. 252/78, 252/498 [51] Int. Cl C09k 3/00 [58] Field of Search 252/78, 79, 76, 75, 565, 252/49.8

[56] References Cited UNITED STATES PATENTS 2,549,270 4/1951 Watson 252/78 2,636,861 4/1953 Watson 252/78 2,636,862 4/1953 Watson 252/78 2,698,837 1/1955 Gamrath 252/78 2,934,501 4/1960 Moreton 252/78 2,999,067 9/1961 Banigan 252/49.8 3,048,545 8/1962 Critchley et a1. 252/78 3,197,408 7/1965 Cupper et a1. 252/5l,5 3,352,783 11/1967 McCord 252/78 3,487,020 12/1969 Peeler et al 252/78 3,513,097 5/1970 Langenfeld 252/78 3,637,507 l/l972 Gentit 252/78v OTHER PUBLICATIONS Encyclopedia of Chemical Technology" Kirk & Othmer, Vol. 8, p. 374, (1965).

Synthetic Lubricant Fluids from Branched-Chain Diesters, Industrial & Engineering Chem., Vol. 39, pp. 484-497, (1947).

Primary Examiner-Mayer Weinblatt Assistant Examiner--Harris A. Pitlick Attorney, Agent, or Firm-Max Geldin [57] ABSTRACT Production of functional fluids for aircraft, industrial and marine use, having good fire resistance, desirable viscosity characteristics, low density and which are relatively inexpensive, especially useful as aircraft hydraulic fluids, comprising a mixture of a phosphorus ester, such as a phosphate, e.g., tributyl phosphate, and a diester of adipic acid or sebacic acid, such as diisodecyl adipate.

7 Claims, No Drawings FUNCTIONAL FLUIDS This invention relates to functional fluid compositions having good fire resistance and desirable viscosity characteristics at both high and low temperatures, and is particularly directed to functional fluid compositions having the above noted properties and consisting essentially of a phosphorus ester, and one or more of certain dicarboxylic acid diesters, particularly a diester of adipic or sebacic acid, the resulting functional fluids being relatively inexpensive while retaining the desirable characteristics and advantages of conventional functional fluids containing chiefly a phosphate ester or mixtures thereof.

Many different types of materials are employed as functional fluids, and functional fluids are utilized in a wide variety of applications. Thus,.such fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluid, power transmission and hydraulic fluids, heat transfer fluids and heat pump fluids. A particularly important application of such functional fluids has been their utilization as hydraulic fluids and lubricants in aircraft, requiring successful operation of such fluids over a wide temperature range, and tire resistant fluids.

Functional and hydraulic fluids employed in many industrial applications and particularly hydraulic fluids for aircraft must meet a number of important requirements. Thus, such hydraulic fluids particularly for aircraft use, should be operable over a wide temperature range, should have good stability at relatively high temperatures and preferably have lubricating characteristics. in addition to having the usual combination of properties making it a good lubricant or hydraulic fluid, such fluid should also have relatively low viscosity at extremely low temperatures and an adequately high viscosity at relatively high temperatures, and must have adequate stability at the high operating temperatures of use. Further, it is of importance that such fluids be compatible with and not adversely affect materials including metals and non-metals such as seals of the system in which the fluid is employed. It is also important in aircraft hydraulic fluids and lubricants that such fluids have as high a flre resistance as possible to prevent ignition if such fluids are accidentally or as result of damage to the hydraulic system, sprayed onto or into contact with surfaces or materials of high temperature.

Another important property particularly for application of a hydraulic fluid in aircraft is the provision of a low density fluid to increase pay load.

Hydraulic fluids in commercial jet aircraft are exposed to temperatures ranging from below -40F to over 200 F. Within these temperature extremes, it is necessary for the fluid to maintain a reasonably low viscosity when cold, and yet not become too thin when hot. As a general rule, this means that the fluid should have a viscosity of less than 1,000 cs (centistokes), and preferably less than 500 cs, at 40F, and maintain a viscosity above 2.0 cs at 2l0F.

Presently available commercial functional or hydraulic fluid base stocks do not possess these viscosity characteristics. Phosphate esters are among the most commonly employed base stocks, of which tributyl phosphate and dibutyl phenyl phosphate are widely used components. Both of the latter phosphates are too thin at high temperatures, and their use alone would result in rapid wear of moving parts. Other phosphate esters,

such as tricresyl phosphate, for example, which provide the requisite high temperature viscosity become too thick to be useful at low temperatures. Even mixtures of various phosphate esters such as those noted above do not provide the required viscosity characteristics at both low and high temperatures. Accordingly, it has been the practice to achieve the required wide viscosity range required foraircraft hydraulic fluids by adding to a thin base stock, such as phosphate ester or mixtures thereof, a small proportion, e.g., up to 10 percent, of a polymeric material such as polyalkyl acrylates or methacrylates, whose solubility characteristics in the base stock are chosen so that the polymeric material thickens the fluid more at high temperatures than at low temperatures, and thus functions as a viscosity index (VI) improver.

However, the chief disadvantage of this method of viscosity improvement is that polymers of the type noted above are generally not stable to the shearing forces encountered in an aircraft hydraulic system, and during use, these large molecules are sheared down to smaller molecules which lose their ability to improve viscosity index. This means that the functional fluid loses viscosity in use and requires addition of polymer to the fluid and eventually, as the detritus from polymeric'breakdown accumulates in the fluid and contaminates it, the fluid must be discarded. In addition, of course, employment of a viscosity index improver in the functional fluid, even in the minor amounts presently employed, substantially increases the cost of the fluid.

It is accordingly an object of this invention to provide a functional fluid which is particularly useful as an aircraft hydraulic fluid and which has requisite viscosity characteristics over a wide temperature range, and can be employed with or without viscosity index improvers, while at the same time having the other requisite properties for a good hydraulic fluid, including good flre resistance and low density, and of substantially reduced cost as compared to presently available aircraft hydraulic fluids, particularly those based essentially on phosphorus esters as the base stock components. Hence it is a particular object to replace such presently employed phosphorus ester base stock fluids, with functional fluids or blends containing base stock components in addition to phosphorus ester or phosphate components, which confer on the functional or hydraulic fluid good viscosity and density characteristics, while substantially reducing the cost of the fluid.

The above objects are achieved according to the present invention by the provision of a functional fluid, particularly valuable as a hydraulic fluid for aircraft, comprising basically a mixture of a phosphorus ester and a dicarboxylic acid diester, particularly a diester of adipic acid or of sebacic acid. The phosphorus ester and diester components are employed preferably in the amounts or proportions set forth hereinafter, in order to obtain the desirable properties of the functional fluid noted above.

Of particular significance, the functional fluids produced according to the invention have a fire resistance comparable to the fire resistance of presently employed commercially available hydraulic fluids, and can be blended to have suitably low viscosity at temperatures below 40F and suitably high viscosity at high temperatures above 210F, and this can be accomplished .with or without employment of a viscosity index improver.

By avoiding the necessity for a viscosity index improver, the functional fluids of the invention do not suffer from the disadvantage noted above, namely, the deterioration of such polymeric additives used for viscosity improvement, and accumulation of molecular debris, leading to a shortening of the useful life of the fluid. Hence the functional fluids of the present invention have a longer period of usefulness, providing economic advantages including the avoidance of the high cost of the viscosity index improver and the employment of relatively low cost diesters such as the above noted adipates and sebacates, in place of a substantial portion of the phosphate ester generally employed in presently available aircraft hydraulic fluids.

Also, the functional fluid base stocks of the present invention have good thermal and hydrolytic stability comparable with the phosphate ester based fluids currently in use, contributing to a long useful life for the fluid. ln addition, the functional fluids according to the present invention have low densities of the order of 1.0 or less, an important property for aircraft hydraulic fluids. The above advantages have been achieved without any sacrifice in the desirable flammability characteristics which are exhibited by the phosphate ester type hydraulic fluids currently in use. Also, the functional fluids according to the invention have freedom from corrosivity and wear with respect to the metallic and nonmetallic components, and the pumps of hydraulic fluid systems, which compare favorably with these characteristics for phosphate type base stock hydraulic fluids presently commercially employed.

Further, in addition to their valuable application as hydraulic fluids for aircraft hydraulic systems, the fluids according to the invention have important application as a hydraulic or functional fluid in industrial and marine fields, particularly in industrial turbine systems.

One of the essential components of the functional fluids according to the invention is a phosphorus ester having the general formula:

where s, m and n can be or I, and not more than two of s, m, and n can be 0, where R R and R each can be aryl such as phenyl and naphthyl, alkaryl such as cresyl, xylyl, ethyl phenyl, propyl phenyl, isopropyl phenyl, and the like, said aryl and aralkyl radicals preferably containing from 6 to about 8 carbon atoms, alkyl, both straight chain and branched chain, of from about 3 to about 10 carbon atoms such as n-propyl, nbutyl, n-amyl, n-hexyl, isopropyl, isobutyl, isoamyl, and the like, and alkoxyalkyl having from about 3 to about 8 carbon atoms such as methoxy methyl, methoxy ethyl, ethoxy ethyl, methoxy propyl, and the like.

The corresponding phosphonates can also be employed, where one of s, m and n is 0, and the corresponding phosphinates where two of s, m and n are 0.

The phosphate esters, that is, where s, m and n are each 1, are the preferred phosphorus esters, and the preferred phosphate esters are the dialkyl aryl, triaryl, trialkyl and alkyl diaryl phosphates.

Examples of such phosphate esters are the dialkyl aryl phosphates in which the alkyl groups are either straight chain or branched chain and contain from about 3 to about 10 carbon atoms, such as n-propyl, nbutyl, n-amyl, n-hexyl, isopropyl, isobutyl, isoamyl, and the aryl radicals have from 6 to 8 carbon atoms and can be phenyl, cresyl or xylyl, particularly dialkyl phenyl phosphates including dibutyl phenyl phosphate, butyl amyl phenyl phosphate, butyl hexyl phenyl phosphate, butyl heptyl phenyl phosphate, butyl octyl phenyl phosphate, diamyl phenyl phosphate, amyl hexyl phenyl phosphate, amyl heptyl phenyl phosphate, and dihexyl phenyl phosphate.

Examples of triaryl phosphates are those in which the aryl radicals of such phosphates have from 6 to 8 carbon atoms, that is, may be phenyl, cresyl or xylyl, and in which the total number of carbon atoms in all three of the aryl radicals is from 19 to 24, that is, in which the three radicals include at least one cresyl or xylyl radical. Examples of such phosphates include tricresyl, trixylyl, phenyl dicresyl, and cresyl diphenyl phosphates.

Examples of trialkyl phosphates employed according to the invention include phosphates having alkyl groups which are either straight chain or branched chain with from about 3 to about 10 carbon atoms, such as npropyl, n-butyl, namyl and n-hexyl, particularly tri-nbutyl phosphate, tri(Z-ethyl hexyl) phosphate and triisononyl phosphate, the straight chain alkyl groups preferably containing from 4 to 6 carbon atoms.

Examples of alkyl diaryl phosphates which can be employed to produce the invention compositions include those in which the aryl radicals of such phosphates may have from 6 to 8 carbon atoms and may be phenyl, cresyl or xylyl, and the arkyl radical may have from about 3 to about 10 carbon atoms, examples of which are given above. Examples of the alkyl diaryl phosphates include butyl diphenyl, amyl diphenyl, hexyl diphenyl, heptyl diphenyl, octyl diphenyl, 6- methyl heptyl diphenyl, butyl phenyl cresyl, amyl phenyl xyly, and butyl dicresyl phosphates.

Particularly desirable phosphate esters are tributyl phosphate, dibutyl phenyl phosphate and tricresyl phosphate, and mixtures thereof.

Functional fluid base stocks according to the invention also include phosphonate and phosphinate esters having alkyl and aryl groups corresponding to those defined above with respect to the phosphate esters.

Examples of phosphinate esters to which the invention principles are applicable include phenyl-di-npropyl phosphinate, phenyl-di-n-butyl phosphinate, phenyl-di-n-pentyl phosphinate, p-methoxyphenyldin-butyl phosphinate, tert-butylphenyl-di-n-butyl phosphinate. Examples of phosphonate esters to which the invention is applicable include aliphatic phosphonates such as an alkyl alkenyl phosphonate, e.g., dioctyl isooctene phosphonate, an alkyl alkane phosphonate such as di-n-butyl n-octane phosphonate, di-isooctyl pentane phosphonate, and dimethyl decane phosphonate, a mixed alkyl aryl phosphonate, for example, dioctyl phenyl phosphonate, di(n-amyl) phenyl phosphonate, di(n-butyl) phenyl phosphonate, phenyl butyl hexane phosphonate and butyl bis-benzene phosphonate.

Although a single phosphorus ester may be employed, in many instances it is desirable to employ a mixture of phosphorus esters in order to obtain the desired properties, e.g., viscosity as well as fire resistance, such as mixtures of phosphates, e.g., a mixture of tributyl and tricresyl phosphates, mixtures of phosphates and phosphonates, mixtures of phosphates and phospinates, and the like.

The diesters of adipic and sebacic acid, the second essential component of the functional fluid composition of the invention, are the aliphatic or alkyl diesters of adipic and sebacic acids. Preferably, the diester component or components are the dialkyl adipates and the dialkyl sebacates, which contain alkyl groups, ei ther straight chain or branched chain, containing from about 4 to about 12 carbon atoms, examples of which include butyl, isobutyl, amyl, pentyl, hexyl, isohexyl, nonyl, decyl, and isodecyl groups. Specific illustrative examples of these adipate and sebacate diesters are dihexyl, di(2-ethylhexyl), dioctyl, dinonyl, didecyl and diisodecyl adipates, and the corresponding sebacates. If desired, a mixture of adipate esters, a mixture of sebacate esters, or a mixture of adipate and sebacate esters can be employed.

Generally, a substantial proportion of both the phosphorus ester, particularly phosphate ester, and diester, particularly adipate diester, are employed in the invention composition. Thus, the amount of phosphorus ester or phosphate present in the invention composi- EXAMPLES l 6 Various compositions containing a phosphate ester or esters and a dialkyl adipate were prepared, in accordance with the invention. In these compositions a polyalkyl acrylate viscosity improver was also added, and also an epoxide stabilizer and anti-corrosion and other conventional additives.

The density of the respective compositions and also the viscosity in centistokes at --40F and at 210F, were measured.

The resulting compositions designated Examples 1 to 6 aregiven in the Table below, together with their respective densities and viscosities at 40F and 210F.

TABLE 1 EXAMPLE OF COMPOSlTlONS ()6 by weight) COMPONENTS l 2 3 4 5 6 Tributyl phosphate 56 50 45 35 30 Di nbutyl phcnyl 0 9 13.5 20 30 25 phosphate Di isodecyl adipate 32.5 30 35 30 36 Poly n-hcxyl acrylatc 5 5 5 3 S 2 (av. m.w. 12,000) Epoxidc stabilizer 2.2 2.2 i 5.2 5.2 3.2 5.2 Anti corrosion and 1.8 1.3 1.3 1.8 1.8 1.8

other additives w Density 0.957 0.968 0.968 0.968 0.968 0.981 Viscosity (cs) -40F 600 628 714 682 972 594 tion can range, by weight, from about 35 to about 85 percent, preferably about to about 80 percent, the amount of adipate diester, sebacate diester, or mixture thereof, can range, by weight, from about 15 to about 65 percent, preferably about 20 to about 50 percent.

As previously noted, a feature of the present invention is that in many instances suitable viscosity characteristics both at low temperatures below 40F and at high temperatures above 210F can be achieved in the absence of a viscosity index improver. However, it may be desirable, particularly for improving pump life in a hydraulic system, to incorporate a small amount ranging from 0 to about 10 percent, generally about 2 to about 10 percent, by weight of the composition, of a viscosity index improver. Examples of the latter are polyalkyl acrylates and polyalkyl methacrylates, in which the alkyl groups may contain from about 4 to about 12 carbon atoms, either straight or branched chain, and having an average molecular weight ranging from about 6,000 to about 15,000. Specific examples of such viscosity index improvers are polybutyl methacrylate and poly n-hexyl acrylate, having an average From the Table above, it is seen that the compositions of Examples 1 to 6 containing a phosphate ester, specifically tributyl phosphate, separately or in combination with di n-butyl phenyl phosphate, and a dialkyl adipate, specifically di isodecyl adipate, each had densities less than 1, and ranging from 0.957 to 0.986, and are relatively low density fluids, and have viscosities at 40F ranging from 594 to 972 cs, and at 210F ranging from 2.65 to 4.21 cs, and within the requisite viscosities of less than 1,000 cs at 40F and more than 2.0 cs at 210F, so that such compositions are useful as functional or hydraulic fluids in aircraft.

The compositions corresponding to Examples 1 to 6 of the above Table also have good fire resistance and good thermal stability.

EXAMPLES 7 and 8 The functional fluid compositions according to the invention, having the compositions noted as Examples 7 and 8 in Table 2 below were prepared, and their density and viscosity were measured and noted in Table 2.

TABLE 2 EXAMPLES OF COMPOSITIONS by weight) COM PONENTS 7 8 Tributyl phosphate Di isodecyl adipate Poly n-hcxyl acrylatc (av. m.w. l2,000) Epoxidc stabilizer and other additives From Table 2 above, it is' seen that even in the absence of a viscosity index improver, as illustrated by the composition of Example 8, both the low temperature viscosity at 40F of 670 es, and the high temperature viscosity at 210F of 2.54 are within the above noted permissible parameters of less than 1,000 cs at 40F and more than 2.0 cs at 210F, and hence can function as hydraulic fluids in aircraft. However, when a portion of the di isodecyl adipate of the composition of Example 8 is replaced by 5 percent poly n-hexyl acrylate viscosity index improver, as illustrated by the composition of Example 7, the resulting viscosity at -40F is desirably reduced to 367 es, and the viscosity at 210F is desirably increased to 3.46 over the corresponding viscosities at 40F and 2l0F of the compositions of Example 8.

However, it is noted that the density of the fluid composition of Example 8 in the absence of viscosity index improver is only 0.93, as compared to 0.96 for the composition of Example 7.

EXAMPLES 9 to 17 The compositions of Examples 9 to 17 set forth in Table 3 below are additional examples of functional fluid compositions according to the invention:

TABLE 3 ture of a phosphorus ester,preferably a phosphate ester, and an adipic acid diester and/or a sebacic acid diester, in the form of a dialkyl adipate and/or a dialkyl sebacate, which have requisite viscosity characteristics at temperatures of below 40F and above 210F. permitting their use in aircraft hydraulic systems even in the absence of viscosity index improvers, and which have comparable fire resistance, thermal stability and corrosion and pump wear resistance of conventionally employed phosphate ester base stocks, and also have desirably low density, with the particular virtue that the resulting invention fluids in which a substantial proportion of phosphorus ester in conventionally employed phosphorus ester base stocks is replaced by adipate and/or sebacate diesters, have substantially improved shear stability and substantially reduced cost as compared to the conventional phosphorus ester base stocks.

While I have described particular embodiments of my invention for purposes of illustration, it will be understood that various changes and modifications within the spirit of the invention can be made, and the invention is not to be taken as limited except by the scope of the appended claims.

I claim:

1. A functional fluid composition consisting essentially of, by weight, (a) about 35 to about 85 percent of a phosphate ester in the form of a trialkyl phosphate containing alkyl groups of from about 3 to about 10 carbon atoms, and (b) about to about 65 percent of a diester compound selected from the group consisting of adipic and sebacic acid alkyl diesters, having alkyl groups containing from about 4 to about 12 carbon atoms.

2. A functional fluid composition as defined in claim 1, said trialkyl phosphate being tributyl phosphate.

3. A functional fluid composition as defined in claim 1, said trialkyl phosphate being tributyl phosphate, and wherein said phosphate ester component additionally contains dibutyl phenyl phosphate, said tributyl phos- EXAMPLES OF COMPOSITIONS (percent by weight) COMPONENTS 9 I0 ll l2 l3 l4 15 Tributyl phosphate 40 4O 45 Dihutyl phcnyl phosphate Tricrcsyl phosphate Phcnyl dicrcsyl phosphate Butyl diphcnyl phosphate Dihutyl phenyl phosphonatc Dipcntyl phenyl phosphonatc Phcnyl di-n-hutyl phosphinatc Dit2-cthylhcxyl) adipate Dihexyl udipalc 4O 25 Dioctyl schucatc 4t) Diisodccyl sehacutc 25 phate being present in an amount of about 30 to about percent, and said dibutyl phenyl phosphate being present in an amount of about 9 to about 30 percent, by weight.

4. A functional fluid composition as defined in claim 3, wherein said diester compound is diisodecyl adipate.

5. A functional fluid composition as defined in claim 1, wherein said phosphate ester is present in amount ranging from about 50 to about 80 percent, and said diester compound is present in an amount ranging from about to about 50 percent, by weight.

2, wherein said diester compound is diisodecyl adipate. 

1. A FUNCTIONAL FLUID COMPOSITION CONSISTING ESSENTIALLY OF, BY WEIGHT (A) ABOUT 35 TO ABOUT 85 PERCENT OF A PHOSPHATE ESTER IN THE FORM OF A TRIALKYL PHOSPHATE CONTAINING ALLYL GROUPS OF FROM ABOUT 3 TO ABOUT 10 CARBON ATOMS, AND (B) ABOUT 15 TO ABOUT 65 PERCENT OF A DIESTER COMPOUND SELECTED FROM THE GROUP CONSISTING OF ADIPIC AND SEBACIC ACID ALKYL DIESTERS, HAVING ALKYL GROUPS CONTAINING FROM ABOUT 4 TO ABOUT 12 CARBON ATOMS.
 2. A functional fluid composition as defined in claim 1, said trialkyl phosphate being tributyl phosphate.
 3. A functional fluid composition as defined in claim 1, said trialkyl phosphate being tributyl phosphate, and wherein said phosphate ester component additionally contains dibutyl phenyl phosphate, said tributyl phosphate being present in an amount of about 30 to about 50 percent, and said dibutyl phenyl phosphate being present in an amount of about 9 to about 30 percent, by weight.
 4. A functional fluid composition as defined in claim 3, wherein said diester compound is diisodecyl adipate.
 5. A functional fluid composition as defined in claim 1, wherein said phosphate ester is present in amount ranging from about 50 to about 80 percent, and said diester compound is present in an amount ranging from about 20 to about 50 percent, by weight.
 6. A functional fluid composition as defined in claim 2, wherein said phosphate ester is present in amount ranging from about 50 to about 80 percent, and said diester compound is present in an amount ranging from about 20 to about 50 percent, by weight.
 7. A functional fluid composition as defined in claim 2, wherein said diester compound is diisodecyl adipate. 